The Laser Diode

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Advanced Optics Laboratory The Laser Diode 1 Introduction This set of laboratory experiments is designed to have you become familiar with the properties of laser diodes and to become familiar with optical spectrum analyzers typically used in fiber and laser instrumentation to asses spectral characteristics of sources. In addition you will become familiar with the technique of using an optical grating to stabilize and to tune the frequency of a laser diode. The discussion and figures in this sec
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  Advanced Optics Laboratory The Laser Diode 1 Introduction This set of laboratory experiments is designed to have you become familiar with the properties of laser diodes and to become familiar with optical spectrum analyzers typically used in fiber and laser instrumentation to asses spectral characteristics of sources. In addition you will become familiar withthe technique of using an optical grating to stabilize and to tune the frequency of a laser diode.The discussion and figures in this section are for the Hitachi 633 nm, HL6314MG diode laser.Figure 1 shows the diode laser with a cut-away of the housing where the chip mounts. Figure 2 showsthe laser chip structure, typical chip dimensions, the direction of the forward current, and the radiation pattern. The radiation is produced in the active layer, which is a small fraction of the height of the chip,hence the radiation is diffracted when it emerges from the active region analogously to the diffractionof radiation passing through a narrow slit. The diffraction produces the radiation pattern shown in Fig.2. The polarization of the emitted light is in the plane of the active junction, and therefore parallel tothe short axis of the elliptically patterned light field.Figure 1. Internal structure of a typical diode laser.Figure 2. Chip structure of a typical diode laser.Figure 3 shows the package type and the internal circuit of HL6314MG. Notice that the package includes a built-in photodiode for the power-monitoring purpose. The common pin of the laser diode and the photodiode is usually designated as the ground pin that is connected to the case. Figure 4shows the actual package dimensions and the pin layout. These two figures allow you to determine theDiode Laser Experiment Page 1 of 10  Advanced Optics Laboratory appropriate mechanical mounting configuration and electric connection for the laser diode. Figure 5displays a number of important operating characteristics of the HL6314MG laser diode. The panel inthe upper left corner shows the optical output power versus the forward bias current, measured under various temperatures. You should notice two important things. First, the laser diode output power risessharply after the forward current exceeds a certain value, namely the lasing threshold current (25 mA).Second, the value of the threshold current increases with the rising temperature of the laser diode. Themaximum operating forward current (50 mA) and power output (3 mW) are specified by themanufacturer for each diode laser. The upper right corner in Fig. 5 shows the relationship between theoutput power of the laser diode and the current of the monitoring photodiode (under a reverse bias of 5V). A calibrated external power meter would allow you to verify this relationship. The ellipticity of the emitted light field pattern is shown in the left lower corner of Fig. 5. The right lower corner of Fig.5 shows that the laser diode operates in an increasingly coherent fashion when its output power isincreased, with a more effective suppression of the side-modes and a narrower linewidth of the mainmode.Figure 3. Package type and internal circuit of HL6314MG laser diode.Figure 4. Package dimensions of HL6314MG laser diode.Diode Laser Experiment Page 2 of 10  Advanced Optics Laboratory Figure 5. Important laser diode operating characteristics.The case temperature of the diode laser plays an important role in laser operations and Figure 6shows the various effects. In general, when the case temperature rises, the diode laser operates lessefficiently, with a rising threshold and decreasing slope efficiency (a conversion factor from theinjection current to the actual optical output power). These scenarios are clearly shown in the top two panels in Fig. 6. Case temperature can be used to tune the operating wavelength of the laser diode.However, the wavelength tuning curve is not smooth over a large range of temperature changes, rather it will display a staircase type of tuning characteristics, as shown in the right lower corner of Fig. 6.This is because the laser diode actually jumps to different longitudinal modes when the temperaturechanges over a large range. While it is possible to change the laser wavelength by a couple of nanometers when the case temperature changes by 10 degree for example, the temperature tuningcoefficient for a particular longitudinal lasing mode (without mode-hopping) is usually much smaller.The capability of continuous tuning of the diode laser wavelength without mode-hop is importantfor a number of applications in the AMO physics, including precision spectroscopy, laser cooling, andchemical sensing, etc. Combined adjustment of temperature and injection current can accomplishwavelength tuning to a certain degree, but usually within a fairly limited range. External cavity laser Diode Laser Experiment Page 3 of 10  Advanced Optics Laboratory diodes help solve this problem. Ordinarily, for a solitary laser diode, the laser cavity is formed betweenthe two cleaved facets of the semiconductor chip. The facets are smoother and flatter than anymechanically polished mirror. If there are no coatings on the end surfaces of the laser chip, then thereflectivity R of a surface is given by:  R = n c − n a n c + n a      2 , where n c and n a , are the indices of refraction of the chip and air, respectively. Sometimes a diode laser manufacturer provides a reduced reflectivecoating on the output facet of the chip; therefore, the reflectivity of this facet is less than that calculatedin Exercise 1. When the output of the diode laser is being returned with the first-order diffracted beamfrom an external grating, an external laser cavity is formed between the grating and the far chip facet.This would be particularly true if the grating diffraction efficiency (40% or more) dominates over theresidual reflectivity of the front facet and the external cavity laser will operate more reliably andstably. The laser wavelength can now be tuned by displacing and/or rotating the grating. A carefullydesigned external cavity diode laser can pull the wavelength of the srcinal laser diode by more than10 nm without adjusting the injection current or chip temperature. It can also tune a single longitudinallaser mode by tens or even hundreds of GHz without mode-hop.Figure 6. The effects of case temperature on laser diode operations.Diode Laser Experiment Page 4 of 10
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